A new 9×9 puzzle arrives every day at midnight UTC, the same for everyone. The week sets the difficulty: Monday and Tuesday are easy, Wednesday and Thursday medium, Friday and Saturday hard, and Sunday is expert. A day's level is checked before it is published, against a toolkit of human techniques whose foundations the guide teaches: easy and medium days fall to singles alone, medium with fewer givens and fewer squares open at the start; a hard day runs out of singles early and needs a pointing pair or a naked or hidden pair; and Sunday needs more than a pair, a triple, an X-wing or an XY-wing, further into the solve. Days published before levels were checked keep the level they were given.
Fill each row, column and 3×3 box with the digits 1 to 9. Clashing digits turn red as you go; pencil marks (Notes, or the N key) clear themselves when you place a digit they ruled out. Your progress and time are kept in this browser, so you can leave and come back.
Yesterday's puzzle — and every one before it — stays in the archive with its solution.